1. Field of the Invention
The present invention generally relates to a mold used in the manufacture of semiconductor devices. More particularly, the present invention relates to a mold allowing a fluid molding compound to be introduced into all cavities at nearly the same time and at a nearly constant velocity by forming runners between a pot and all the cavities that have nearly equal distances and are radially oriented from a center point of a main runner.
2. Description of the Related Art
Highly integrated semiconductor devices are characterized by increased memory capacity, electric power consumption, performance speed, and mounting density. As a result, the number of leads connecting the semiconductor device package to an external apparatus has increased, and the leads are finer and more difficult to form. Therefore, the molding step in which the semiconductor chips, inner leads, and the electrical interconnections are encapsulated has become more important than ever.
Molding compounds which are used in the molding step must have particular characteristics consistent with these finer and denser contacts between the chip and lead frames. Such molding compound characteristics as thermal resistance, mechanical strength, thermal conductivity, hardness, abrasiveness, thermal expansion coefficient, and so on, must meet the characteristics of the chips, leads and bonding wires. Encapsulants, such as epoxy molding compound (EMC), form a package body and provide protection from external environmental stresses. The Transfer Molding Method is frequently employed for the encapsulation process. When circumstances are especially delicate, as in the case of electrical interconnections using a bonding wire as thin as about 23 .mu.m to about 38 .mu.m in diameter, a Low Pressure Transfer Molding Method is employed. Using this method, the bonding wire is not damaged during the molding step.
FIG. 1A is an cut-away perspective view of a conventional one-pot type molding device, and FIG. 1B is an enlarged perspective view of the portion `A` from FIG. 1A.
Referring to FIG. 1A and FIG. 1B, a conventional molding device 100 includes a press 40 and a mold 70. The mold 70 consists of an upper mold 50 and a lower mold 60 which is separably coupled to the upper mold 50.
The press 40 has a support plate 20 which is mechanically connected to a vertically movable transfer means (not shown) and a cylinder 10 fixed to an upper surface of the support plate 20. The cylinder 10 contains hydraulic fluid 35 provided through a hydraulic inlet port 12 at the top of the cylinder and discharged through an outlet port 14 below the inlet, and it contains the upper part of a press rod 30. The lower part of the press rod 30 extends downward, below the cylinder 10.
The lower mold 60 is mounted onto and fixed to an upper surface of a base part (not shown) which is located at the lowermost part of the molding device 100. The lower mold 60 comprises a receive part 62 for receiving molding compound tablets, a plurality of runners 65 in flow communication with the receive part 62, a plurality of gates 66 which are connected to the runners 65, and a plurality of cavities 68 which are connected to the gates 66. The semiconductor package assembly, having electrical interconnections between a chip and a lead frame unit, is encapsulated within each cavity 68. In the mold, the runners 65 and gates 66 are essentially passages through which the molding compound passes, and the cavities 68 are the destinations where package bodies are formed from the molding compound.
The upper mold 50 is mechanically coupled to a vertically movable transfer means (not shown) at its upper surface or other parts, and has a pot 52 where the molding compound is loaded as tablets. The upper mold 50 has a runner structure symmetrical to that of the lower mold 60, except the upper mold 50 does not include a counterpart to the gates 66 of the lower mold 60.
For the lower mold 60, a set of runners 65 is composed of a main runner 64 in direct flow communication with the receive part 62, and a plurality of sub-runners 63 which extend from the main runner 64 and are in flow communication with the cavities 68 through the gates 66. There are several sets of runners in each lower mold 60. In the structure of the runners 65 of the conventional mold 70, the sub-runners 63 are substantially perpendicular to the main runner 64.
FIG. 2A is a cut-away perspective view depicting strips 90 of lead frame units within a lower mold of the molding device of FIG. 1A, and FIG. 2B is an enlarged perspective view of the portion `B` from FIG. 2A. FIG. 3 is a cut-away perspective view for depicting the package assemblies after the molding step is completed. With reference to FIG. 2A, FIG. 2B and FIG. 3, the molding step and the operation process of the molding device 100 are described.
Lead frame strips 90 are placed onto the cavities 68 of the lower mold 60. The lead frame strips 90 comprise a plurality of lead frame units. Each lead frame unit has a chip 91, inner leads 93, and electrical interconnection means such as bonding wires 92, that connect the inner leads to the chip. The upper mold 50 is lowered by the transfer means (not shown), which is mechanically coupled to the upper mold 50, until the bottom surface of the upper mold 50 touches the upper surface of the lower mold 60. Thereafter, the molding compound tablets are introduced into the pot 52 of the upper mold 50 and receive part 62 of the lower mold 60.
The press 40 is lowered until the press rod 30 touches the upper surface of the tablets. The lowering of the press 40 is accomplished by the transfer means which is mechanically connected thereto. At this time, the tablets are melted by preheating the upper and lower molding dies 50 and 60, respectively, to a temperature in the range from about 170.degree. C. to 180.degree. C. or by using a separate tablet-preheating device. The hydraulic fluid 35 is then pressurized through the hydraulic inlet port 12 in the cylinder 10 which lowers the press rod 30. The lowered press rod 30 presses the fluid molding compound (FMC) 80 so that the FMC 80 flows from the pot 52, into the receive part 62, the runners 65, the gates 66, and the cavities 68. In each cavity 68 a lead frame unit from the lead frame strip 90 is encapsulated so that the chip 91, the inner leads 93, and the electrical interconnection means such as the bonding wires 92 are engulfed by the FMC 80. After the molding step is completed, the press 40 is elevated by the transfer means and the flow of the FMC is stopped. The FMC then hardens into the package body of the semiconductor package 95. Then the upper mold 50 is elevated by the transfer means and the lead frame strips 90 formed into package assemblies 95 are removed from the lower mold 60.
Because the bonding wires 93 are easily damaged by high viscosity fluids, the viscosity of the FMC 80 is usually as low as 200.about.500 PS (poises). The viscosity of the FMC increases with temperature and process time as the molding compound hardens; and the gelation point is the viscosity above which the molding compound ceases to move as a fluid. To reduce encapsulation process time, it is desired that the FMC be moved through the runners at a velocity sufficient to fill the cavities within 20 to 30 seconds while the FMC is still below the gelation point.
FIG. 4 is a graph depicting the state change of the molding compound during the molding step using the molding device of FIG. 1. Referring to FIG. 2A through FIG. 4, the relationship between the state of the molding compound and the molding process is described. The horizontal axis represents the time during the molding step, starting with the loading of the tablets up until the molding step is accomplished, in arbitrary units. The vertical axis represents the change of viscosity of the molding compound during the molding step. A point `A` denotes viscosity of the molding compound tablets at the start of the molding step and a point `B` denotes a point at which the molding compound tablets start melting to form the FMC 80 within the pot 52 of the mold 70. Between points `C` and `E` viscosities of the FMC are low enough to flow into the cavities 68 without damaging the bonding wires 92, i.e., the viscosity of point `C` represents a safe level. A point `D` denotes a point having the lowest viscosity. The viscosities below the safe level of the viscosity of `C` and above viscosity of `D` correspond to a working viscosity range in which the tablets are melted and flow readily into the respective cavities 68 within the mold 70 without damage to the bonding wires 92. The time interval between points `C` and `E`, represents a safe time interval during which FMC can flow into the cavities 68 without damage to the bonding wires 92.
Before the point `D`, the viscosity is dropping gradually with the melting of the tablets, and after the point `D`, the viscosity is rising gradually due to the hardening of the molding compound with increasing temperature and process time. After the gelation point `F`, the FMC loses its fluidity, and the encapsulation is completed. At point `G` the lead frame strips are separated from the mold. At point `H` the molding compound is essentially completely hardened and thereafter the state does not change significantly.
Thus, it is desirable for the molding step to be accomplished during the safe time interval between points `C` and `E` so that the semiconductor chip, the inner leads and the bonding wires are not damaged by the flow of the FMC. Though it performs well in many ways, the conventional molding device 100 suffers from being unable to fill all the cavities completely during the safe time interval. The reasons for this deficiency are described next.
FIG. 5 depicts the result of numerical simulations for flow of the FMC in the conventional molding device 100. With reference to FIG. 3 through FIG. 5, the lowered press rod 30 compresses the FMC 80 from the molten tablets and causes the FMC 80 to flow into the pot 52, the main runner 64, the sub-runners 63, and the cavities 68. In the conventional structure 100 where the sub-runners 63 are perpendicular to the main runner 64, there is a substantial pressure drop in the main runner 64 from the location where the first sub-runner 63, closest to the pot, joins the main runner 64 at the first proximal end of the first sub-runner 63, to the location where the last sub-runner 63, farthest from the pot, joins the main runner 64 at the proximal end of that last sub-runner 63. Therefore the FMC 80 fills the cavities 68 beginning with the first cavity 68a closest to the pot 52 with greater force than the last cavity 68b farthest from the pot 52. Hereinafter, the pair of the cavities closest to the pot 52 are the first cavities 68a and a pair of the most distant cavities from the pot 52 are the last cavities 68b.
A transfer time is the time it takes for the FMC 80 to traverse from the pot 52 to a cavity 68. The difference between the transfer time when the FMC 80 fills the first cavity 68a and the transfer time when the FMC fills the last cavity 68b is the fill time interval, i.e., the time interval needed to fill all the cavities 68. It is preferable that the transfer times for all the cavities 68 occur in the safe time interval between points `C` and `E` and most preferably that all the transfer times occur near the time around the point `D` when the FMC 80 has its lowest viscosity.
However, because the sub-runners 63 and the main runner 64 are perpendicularly connected, if the number of the cavities 68 along one main runner 64 from one pot 52 increases, the distance between the first cavity 68a and the last cavity 68b is lengthened, the transfer times and fill times increase, and the FMC 80 flows into the last cavity at a time beyond the `C` to `E` safe time interval. That is to say, the bonding wires 92 in the more distant cavities 68, including the last cavity 68b, are damaged by the flow of the FMC having high viscosity.
Even with a viscosity in the working viscosity range below the safe level, the bonding wires 92 can be damaged if the FMC 80 enters the cavity 68 at a velocity that is too high and voids can be created if the FMC 80 enters the cavity 68 at a velocity that is too low. The velocity of FMC entering a cavity 68 is related to the rate at which the cavity fills, i.e., the filling rate. FIG. 6 is a graph depicting the respective filling rates of the FMC, in terms of percent filled as a function of encapsulation time (rendered dimensionless by dividing by encapsulation time).
Referring to FIG. 6, a solid-lined curve 78a having a decreasing slope with time depicts the filling rate of the FMC 80 into the first cavity 68a and another solid-lined curve 78b having an increasing slope with time depicts the filling rate of the FMC 80 into the last cavity 68b. The slopes of the curves 78a and 78b represent the velocity profiles of the FMC 80 in the first and last cavities, respectively.
In the first cavity 68a, the FMC 80 starts with a high velocity while the pressure is high, and loses velocity as the cavity 68a fills. By contrast, in the last cavity 68b the FMC 80 starts filling when the pressure is low and thus has a low velocity at the start, then the FMC gains speed as the cavity 68b fills.
More specifically, in the conventional mold, the lowered press rod 30 causes the FMC to flow along the main runner 64. At the beginning, the pressure is greatest at the proximal end of the first sub-runner feeding the first cavity 68a, and the FMC rapidly fills the first cavity 68a closest to the pot. But, the pressure is still low at the proximal end of the last sub-runner feeding the last cavity 68b, thus, the FMC begins filling the last cavity 68b at a very low velocity. Therefore the last cavity 68b will have voids due to lower velocities during the start of the cavity's filling. That is, there is a large pressure gradient in the main runner 64 at the start of the process. The cavities more distant from the pot are less affected by the pressure of the press rod 30 on the FMC. The sub-runners 63 are perpendicular to the main runner 64, and thus the proximal ends of the sub-runners 63 branch off from the main runner 64 at very different distances along the main runner 64. At these different distances, therefore, the pressures created in the main runner 64 by the press rod 30 are very different. The last cavities feel less pressure and fill at a lower velocity at the start. At the end of the molding step, when the closer cavities are filled, there is less of a pressure drop from the pot to the distant cavities, the pressure is higher at the proximal end of the last sub-runner feeding the last cavity 68b, and the last cavity 68b fills at a higher rate. Thus in the last cavity 68b, the velocity at the end of the molding period is highest, as shown by the curve 78b in FIG. 6.
Thus a problem with the conventional molding device 100 is the large distances between where the proximal ends of the sub-runners 63 intersect the main runner 64, which leads to large differences in pressure, and thus to larger changes in velocity of FMC filling the cavities 68. The greater velocities can lead to damage of the bonding wires 92 and the lower velocities can lead to voids in the package body.
The other problem with the runners of the conventional mold device is that the FMC is sequentially introduced into the cavities 68 starting with the upper cavity 68a. The sequential introduction is a problem because the last cavity 68b is filled much later than the first cavity, close to or after the safe time interval, and the gate of the last cavity 68b is more easily blocked due to the increased hardening of the FMC at that time.
The straight line in FIG. 6 between the curve 78a of the first cavity 68a and the curve 78b of the last cavity 68b is an ideal filling line 79 which indicates an ideal filling rate. The ideal filling line 79 shows that the velocity of the FMC is constant and all the cavities are filled without voids and without damage to the bonding wires. The closer the filling curves 78a and 78b approximate the ideal filling line 79, the more ideal are the molds created in the cavities 68. The filling curves of the conventional mold deviate significantly from the ideal filling line.
Therefore, there is a need for a mold that will fill the first and last cavities closer together in time, and with smaller deviations in velocity, requiring a smaller drop in pressure, than is possible with the conventional molds.